A replacement window installed in the existing jamb (frame) is typically installed this way: After cleaning the opening and inspecting it for water leaks, etc. (and correcting such issues), a bead of caulk is applied to the interior of the blind stop. The window is then placed in the opening, sealing the outside edge against the blind stop. The foam or fiberglass insulation (we use foam) is then inserted from inside the window frame. The interior stops are then installed. If there is a gap in the alignment of the stops, a thin caulk line is applied (paintable caulk) to seal that unsightly gap and prepare it for future painting. The exterior is then enclosed with aluminum trim sheet (formed in a brake) and caulked to make it weatherproof. The major mistake some installers make is to measure too close, not allowing room for an insulating product to fit between window jamb and frame. This always causes poor performance and air leaks. We always measure to allow 1/8" gap around the perimeter for insulation product. Michael

Sounds too cheap to me. Two high quality casement windows alone will probably be $1,000 to 1,200.00 plus other materials and a solid day and a half for two guys and other materials. What brand window is he installing? What type "contractor" is he? Licensed and insured?

+1 to Todd's comments. Its not an issue. Does vinyl expand and contract more, yes. Do the product engineers know this and design the window accordingly, yes. Is it basically a trumped up idea to sell more fiberglass windows, also yes.... Fiberglass windows can be very nice don't get me wrong, especially some of those of Canadian origin that boast some great performance ratings. Buy them for that reason or another like color choices, not based on the bogus "expansion and contraction" narrative in comparison to vinyl.

Somewhere I saw a house that had a long, 12" to 15" wide wire-topped table under its first floor windows. It was set with lots of red geraniums in same-size pots. Very pretty. Easy to water because of the wire, and it would be easy to replace an individual plant if needed. So, not a true "window box" but it avoids the "how to attach it" issue.